Microscopic processing often requires a small quantity of a compound to be provided at a precise location on a work piece. For example, vapor phase precursor molecules directed to a region of a work piece dissociate in the presence of a focused beam, such as a charged particle beam or a laser beam, to deposit material or to etch a work piece in a precise pattern. Stains or markers applied to a region of interest on a work piece surface may increase the contrast of structures in a biological system for imaging with an electron or optical microscope. An electrolyte applied to the surface of the work piece may be used for local electrochemical deposition or etching of a material. Reactants may combine in a microscopic reactor on a work piece to produce a desired product or to test for the presence of a substance.
Charged particle beams, such as focused ion beams or electron beams, and laser beams are often used with chemical compounds to process a work piece. For example, beams can be used to deposit material by beam-induced deposition, also known as “direct-write deposition.” One method of direct-write deposition uses electron beam, ion beam, or laser beam-stimulated chemical vapor deposition, in which a precursor species dissociates due to the effects of the beam. Part of the dissociated molecule is deposited onto the substrate, and part of the dissociated molecule forms volatile by-products, which eventually releases from the work piece surface. The precursor can be, for example, a vapor that contains an organometallic material that includes a metal to be deposited. The metal is essentially deposited only in the area impacted by the beam, so the shape of the deposited metal can be precisely controlled with resolution close to that of the beam. The precursor is typically delivered to the work piece surface as a vapor using a needle that directs the precursor gas to the vicinity of the work piece where the beam impacts. An ion beam-assisted deposition process is described, for example, in U.S. Pat. No. 4,876,112 to Kaito et al. for a “Process for Forming Metallic Patterned Film” and U.S. Pat. No. 5,104,684 to Tao et al. for “Ion Beam-Induced Deposition of Metals.”
Beam-induced processes can also be used to precisely etch a work piece, with the beam inducing a reaction between a precursor compound and a material on the work piece to form a volatile compound that leaves the surface. Because charged particle beams can be focused to a spot smaller than one tenth of a micron, charged particle beam processes provide for high-resolution fabrication, alteration, and imaging of microscopic structures. Charged particle beams operate in a vacuum, while lasers can operate either in atmosphere or in a vacuum.
Charged particle beams can also be used to form images and to study the properties of microscopic structures, for example, in transmission electron microscopy, scanning electron microscopy, scanning ion microscopy, energy dispersive electron spectroscopy, and Auger electron spectroscopy. Delivering compounds, such as stains and markers, to a precise location on a work piece can be useful in imaging and metrology. Markers may include heavy elements that provide improved contrast in electron beam images. Recent advances in super-resolution optical microscopy provide high-resolution images of biological structures, often in conjunction with markers, such as fluorescent proteins. An example of the use of a substance to enhance imaging of a work piece is shown in U.S. Pat. No. 7,977,631 to Mulders et al. for “Method for obtaining images from slices of specimen,” which describes the use of staining with charged particle beam processing and imaging. In Mulders, a gas-phase stain is delivered to a freshly exposed surface of the work piece.
Processing micrometer and nanometer scale structures is required in many fields including biological sciences, microelectromechanical systems (MEMS) and semiconductor manufacturing. For example, semiconductor devices such as microprocessors can be made up of millions of transistors, each interconnected by thin metallic lines branching on several levels and electrically isolated from each other by layers of insulating materials. Biological sensors may include microscopic regions of biological material that detect an analytic, transducers and electronics that provide an interpretable detectable signal.
One application of beam processing is device editing—the process of modifying a device during its development without having to remanufacture the whole circuit. Device editing provides tremendous economic benefits by reducing both processing costs and development cycle times. Direct-write deposition and precise etching allows an engineer to test variations of the device without undertaking the lengthy process of modifying photolithography masks and fabricating a new circuit from scratch.
It is often difficult to obtain high purity materials using direct-write deposition, primarily due to the incorporation into the deposit of other components of the precursor molecules or the elements from the incident ion beam, such as gallium ions. This lack of control of composition, material purity, or internal structure often leads to undesirable properties in the deposited material. Tungsten and platinum deposited by focused ion beam (FIB)-induced deposition typically have resistivities greater than about 150 micro ohm centimeters (μΩ-cm). Recently-introduced FIB copper depositions have resistivities of 30-50 μΩ-cm. This is significantly higher than the resistivity of pure copper, which is less than 5 μΩ-cm.
U.S. Pat. No. 7,674,706 to Gu et al. for “System for Modifying Structures Using Localized Charge Transfer Mechanism to Remove or Deposit Material” (“Gu”) describes local electrochemical processing. The process of Gu provides improves purity and lower resistivity compared to ion beam-induced deposition. Gu deposits a localized drop of electrolyte on a portion of an integrated circuit and depositing or etching using an electric current flowing from a probe contacting the drop, through the electrolyte and then through the substrate. In one embodiment, the probe contacting the drop is replaced by using a charged particle beam to supply current, with the circuit being completed through the substrate.
Charged particle beam-induced deposition has been limited by the availability of vapor phase precursors with requisite properties, that is, high residency time (stickiness) on the surface, lack of spontaneous decomposition, and decomposition in the presence of the beam to deposit the desired material and form a volatile byproduct. When suitable deposition precursors do exist for a particular material, the deposition rates are often limited by gas depletion effects and other factors. The precursor molecules typically cause a substantial increase in the pressure in the vacuum chamber, which can scatter the beam, and most of the precursor molecules are removed by the vacuum pump without reacting, thereby wasting the precursor, which is often a hazardous substance.
FIG. 1 shows an apparatus for localized electrochemical deposition of conductors using a micro or nano pipette in close proximity to a conductive surface. Such a method is described in Suryavanshi et al. in “Probe-based electrochemical fabrication of freestanding Cu nanowire array,” Applied Physics Letters 88, 083103 (2006) (“Suryavanshi”). A glass pipette 102 holds an electrolyte solution 104, such as 0.05 M CuSO4. A power supply 106 provides current for the electrochemical reaction, with an electric circuit being formed between a copper electrode 108 and a conductive substrate 110. The process is typically carried out in atmosphere under the observation of an optical microscope. A device that moves about a surface writing a pattern is referred to as a “nano pen.”